In an attempt to improve the sensitivity and linearity of the MR effect, it is known to apply a constant bias magnetic field to an MR head. This field rotates the magnetization of an MR sensor film to a new direction at an angle .theta. with the original orientation direction. This rotation of the magnetization can be achieved in the following ways:
(1) Locate a permanent magnet adjacent the MR sensor film. However, this approach provides linearity only adjacent the top and bottom edges and not over the entire height of the MR sensor film. Moreover, if the permanent magnet is placed directly over the active area of the MR sensor film, the coercivity of the soft film may be raised to a level that is undesirable for sensor applications.
(2) Position a shunt conductor in contact with the MR sensor film. However, this provides poor sensitivity because the conductive film short circuits the MR film, and thereby removes at least 50% of the signal.
(3) Locate a soft magnetic film adjacent the MR film. This provides good linearity; but it does not provide good sensitivity because the soft film shunts the current away from the MR sensor film and thereby can remove about 30% of the signal.
(4) Asymmetrically position the MR film in the gap between the magnetic shields, with or without a shunt layer contacting the MR film, as taught in U.S. Pat. No. 3,940,797. This will not, without the shunt layer, provide a bias field that can achieve an adequate degree of linearization unless current density is increased to a level where power dissipation overheats and destroys the sensor. Moreover, for thin gaps of the order of 2 .mu.m, it is difficult if not impossible to manufacture an MR head that will provide sufficient asymmetry to create a bias magnetic field of meaningful magnitude. This is because the bias field magnitude increases with degree of asymmetry; and with a ratio of over 3:1, as taught by this cited patent for MR heads without a shunt layer, the MR film would he spaced so close to one of the magnetic shields that electrical shorting would very likely occur.
(5) Create a magnetically biased MR strip by oxidizing portions adjacent each side edge of a Permalloy strip sensor so that they become hard magnets while the unoxidized central portion remains soft, as described in connection with FIG. 4 of U.S. Pat. No. 3,840,898. However, this technique can raise the coercivity of the Permalloy material only about 50-100 Oe. This is insufficient to prevent the oxidized regions from becoming demagnetized near their edges. When this occurs, the magnetization in the oxidized regions will not remain canted and hence not provide the desired bias. Finally, as a result of this oxidation, the magnetization at the oxidized side portions will undesirably be less than that at the central portion.
There is a need for a magnetic storage system having an MR head that has optimal transverse bias and high read efficiency and is especially suitable for narrow track width applications.